This invention relates to processes for the production of thermoplastic compositions based on vinyl chloride polymers and polymers for impact resistance modification as well as thermoplastic molding compositions produced thereby which exhibit, in addition to high impact resistance and good surface gloss, high transparency and weatherability.
Polyvinyl chloride (PVC) is a synthetic resin widely utilized because of its good properties and favorable price. When PVC is employed alone, it exhibits high transparency, but inadequate impact strength. In order to improve impact strenth, vinyl chloride polymers are treated with a great variety of modifiers. Examples include polymeric modifying agents of the butadiene type, such as acrylonitrile-butadiene-styrene (ABS) and methyl methacrylate-butadiene-styrene (MBS); copolymers of ethylene with vinyl acetate (EVA); chlorinated polyolefins, such as chlorinated polyethylene (CPE); ethylene-propylene rubbers and polymers of the acrylate type, such as homo- and copolymers of acrylic acid alkyl esters. However, although these modifiers do increase impact resistance, they greatly impair, depending on the type of modifier employed, other desirable properties of PVC, e.g., transparency or weatherability.
Consequently, there have been many attempts to increase the impact resistance of polyvinyl chloride without impairing the other properties mentioned above.
U.S. Pat. No. 3,763,279 (DOS No. 2,222,876), which disclosure is incorporated by reference herein, discloses the production of multiple-layer polymers exhibiting a hard inner core (e.g., crosslinked polystyrene, index of refraction higher than that of PVC) and a rubber-elastic outer layer (e.g., a crosslinked acrylic ester polymer; index of refraction smaller than that of PVC). The composition of the core/shell polymer must be chosen in correspondence with the desired properties. For high transparency, the styrene proportion should be about 55%, and for high impact resistance below 50%. Manufacture takes place in two successive emulsion polymerizations. In a third step, covering polymerization with vinyl chloride in emulsion or suspension is employed. However, the resultant molding compositions exhibit, however, a poor surface gloss and hazy portions. Moreover, the use of a styrene seed latex is required.
According to the patent, special conditions must be observed during production. The disclosure mentions th necessity of destroying the catalyst residues and driving out the unpolymerized monomers, after production of the polystyrene seed latex, as well as after the first covering polymerization with acrylic acid ester. This avoids the formation of new particles in subsequent stages since they decrease transparency. These intermediate steps, however, are time and energy consuming. The grain obtained after the covering polymerization with vinyl chloride is non-porous. In order to circumvent ensuing difficulties during drying, it is recommended to mix the suspension with a modifier-free PVC suspension and process them together. However, without high porosity, ready devolatizing ability and thus economically advantageous and physiologically harmless production and further processing, and use of the vinyl chloride polymers (EP No. 0 033 893) is not attained. Also the resultant polymers do not demonomerize at lower temperatures. The ability to demonomerize at lower temperature results in less thermal damage to the vinyl chloride polymer.
DAS No. 2,557,828, corresponding to Canadian No. 1,063,285, describes the production of a modifier in powder form for use in impact-resistant and transparent molding compositions based on vinyl chloride polymers.
In the first stage, a mixture of an aromatic vinyl monomer is polymerized with a crosslinking agent, and this polymer is covered, in a second stage, by a mixture of an acrylic acid alkyl ester with a crosslinking agent by polymerization. In a third stage, methyl methacrylate and an aromatic vinyl monmer are then applied by polymerization onto the polymer obtained in the second stage. All polymerization stages are performed in emulsion. The multiple-layer polymer is coagulated by salting out and is washed, dryed, and preferably pulverized. This pulverized multiple-layer polymer is incorporated into polyvinyl chloride with the optional addition of customary auxiliary agents.
In this patent, the showing is made, with the aid of examples, that a direct covering polymerization of vinyl chloride onto the modifier obtained in the second stage leads to products having markedly poorer properties. Furthermore, the isolation of the modifier is accompanied by additional work and a burdening of the wastewaters with salt.
However, contrary to the teachings of this patent, direct grafting has been found desirable, since improved binding of the modifier particles to the PVC matrix is achieved thereby.
Also, in such a case, no hard shell material (which material does not contribute toward improvement of the mechanical properties) is introduced into the vinyl chloride polymer.